The invention generally relates to medical apparatus and methods. More specifically, the invention relates to apparatus and methods for treatment of a patent foramen ovale.
Fetal blood circulation is much different than adult circulation. Because fetal blood is oxygenated by the placenta, rather than the fetal lungs, blood is generally shunted away from the lungs to the peripheral tissues through a number of vessels and foramens that remain patent (i.e., open) during fetal life and typically close shortly after birth. For example, fetal blood passes directly from the right atrium through the foramen ovale into the left atrium, and a portion of blood circulating through the pulmonary artery trunk passes through the ductus arteriosis to the aorta. This fetal circulation is shown in FIG. 1.
At birth, as a newborn begins breathing, blood pressure in the left atrium rises above the pressure in the right atrium. In most newborns, a flap of tissue closes the foramen ovale and it heals together. In approximately 20,000 babies born each year in the United States, the flap of tissue is missing, and the hole remains open as an atrial septal defect (ASD). In a much more significant percentage of the population (estimates range from 5% to 20% of the entire population), the flap is present but does not heal together. This condition is known as a patent foramen ovale (PFO). Whenever the pressure in the right atrium rises above that in the left atrium, blood pressure can push this patent channel open, allowing blood to flow from the right atrium to the left atrium.
Patent foramen ovale has long been considered a relatively benign condition, since it typically has little effect on the body's circulation. More recently, however, it has been found that a significant number of strokes may be caused at least in part by PFO. In some cases, stroke may occur because a PFO allows blood containing small thrombi to flow directly from the venous circulation to the arterial circulation and into the brain, rather than flowing to the lungs where the thrombi can become trapped and gradually dissolve. In other cases, thrombi might form in the patent channel of the PFO itself and become dislodged when the pressures cause blood to flow from the right atrium to the left atrium. It has been estimated that patients with PFOs who have already had cryptogenic strokes may have an increased risk of having another stroke.
Research is currently being conducted into the link between PFO and stroke. At the present time, if someone with a PFO has two or more strokes, the healthcare system in the U.S. may reimburse a surgical or other interventional procedure to definitively close the PFO. It is likely, however, that a more prophylactic approach would be warranted to close PFOs to prevent the prospective occurrence of a stroke. The cost and potential side-effects and complications of such a procedure must be low, however, since the stroke event rate due to PFOs is relatively low. In younger patients, for example, PFOs sometimes close by themselves over time without any adverse health effects.
Another highly prevalent and debilitating condition, chronic migraine headaches, has also been linked with PFO. Although the exact link has not yet been explained, PFO closure has been shown to eliminate or significantly reduce migraine headaches in many patients. Again, prophylactic PFO closure to treat chronic migraine headaches might be warranted if a relatively non-invasive procedure were available.
Currently available interventional therapies for PFO are generally fairly invasive and/or have potential drawbacks. One strategy is simply to close a PFO during open heart surgery for another purpose, such as heart valve surgery. This can typically be achieved via a simple procedure such as placing a stitch or two across the PFO with vascular suture. Performing open heart surgery purely to close an asymptomatic PFO or even a very small ASD, however, would be very hard to justify.
A number of interventional devices for closing PFOs percutaneously have also been proposed and developed. Most of these devices are the same as or similar to ASD closure devices. They are typically “clamshell” or “double umbrella” shaped devices which deploy an area of biocompatible metal mesh or fabric (ePTFE or Dacron, for example) on each side of the atrial septum, held together with a central axial element, to cover the PFO. This umbrella then heals into the atrial septum; the healing response forming a uniform layer of tissue or “pannus” over the device. Such devices have been developed, for example, by companies such as Nitinol Medical Technologies, Inc. (Boston, Mass.) and AGA Medical, Inc. (White Bear Lake, Minn.). U.S. Pat. No. 6,401,720 describes a method and apparatus for thoracoscopic intracardiac procedures which may be used for treatment of PFO.
Although available devices may work well in some cases, they also face a number of challenges. Relatively frequent causes of complications include, for example, improper deployment, device embolization into the circulation, device breakage, and device erosion where constant rubbing of the metal frame erodes adjacent tissue resulting in collateral tissue damage. In some instances, a deployed device does not heal into the septal wall completely, leaving an exposed surface which may itself be a nidus for thrombus formation. Furthermore, currently available devices are generally complex and expensive to manufacture, making their use for prophylactic treatment of a PFO impractical. Additionally, currently available devices typically close a PFO by placing material on either side of the tunnel of the PFO, compressing and opening the tunnel acutely, until blood clots on the devices and causes flow to stop. A few methods and devices close a PFO by inserting a device primarily into the tunnel of the PFO to cause closure.
Research into methods and compositions for tissue welding has been underway for many years. Of particular interest are technologies developed by McNally et. al., as shown in U.S. Pat. No. 6,391,049 and Fusion Medical, as shown in U.S. Pat. Nos. 5,156,613, 5,669,934, 5,824,015 and 5,931,165. These technologies all disclose the use of energy delivery to tissue solders and patches in order to join tissue and form anastomoses between arteries, bowel, nerves, etc. Also of interest are a number of patents by inventor Sinofsky, relating to laser suturing of biological materials (e.g., U.S. Pat. Nos. 5,725,522; 5,569,239; 5,540,677 and 5,071,417). Other references, such as PCT Patent Application Publication No. WO 03/0534493, describe devices for closing PFOs involving bioresorbable materials. Other PCT Patent Application Publications such as WO 99/18871 and WO 99/18870 describe radiofrequency balloon catheters delivering energy within a PFO tunnel to close the defect, and U.S. Patent Application Publication Nos. 2003/0208232 and 2006/0036284 describe applying a lateral force to a PFO tunnel in order to close the tunnel. While these basic technologies may be applicable to the closure of PFOs, none of these disclosures, however, show methods or apparatus suitable for positioning the tissues of the PFO for welding or for delivering the energy to a PFO to be welded when a portion of the surfaces of the layers of the defect are in contact. These references also do not describe applying a force to the layered tissue defect having a portion of the surfaces of the layers in contact, so as to bring the layered tissue defect together.
Therefore, it would be advantageous to have improved methods and apparatus for treating a PFO. Ideally, such methods and apparatus would help seal the PFO while minimizing any repair material left behind or preferably leaving very little or no foreign material, in the body. Also ideally, such methods and apparatus would be relatively simple to manufacture and use, thus rendering prophylactic treatment of PFO, such as for stroke prevention, a viable option. At least some of these objectives will be met by the present invention.